Car crash reaction time

Question

In physics class we are doing an investigation thing. Some of the numbers might seem plucked out of thin air but they aren't. The maths is right(I think) I just want some feedback on the code.

import datetime
import random
import time
import math


player_1 = input("Player one: ")
speed = int(input("Speed in km/h: "))
speed_ms = ((speed)/3.6)
conditions = input("Wet or dry conditions? (all lowercase)")

if conditions == "wet":
    friction_coefficient = 0.4
else:
    friction_coefficient = 0.6


time.sleep(0.5)
print("Get ready..")
time.sleep(random.randint(1,12))
then = datetime.datetime.now()
t = input("GO!! ")
now = datetime.datetime.now()
diff = then-now
reaction_time = round(abs(diff.total_seconds()), 2)


Rt = (math.sqrt((reaction_time*98)/490))
print('Your final reaction time was: ', (Rt), '!\n')

Rd = (Rt * speed_ms)
print('Your final reaction distance was: ', (Rd), '!\n')

Bd = (((speed_ms)**2) / 19.6 * (friction_coefficient))
print('Your final breaking distance was: ', (Bd), '!\n')

Sd = ((Rd)+(Bd))

############################################################################

print("\n\n---===========FINAL STOPPING DISTANCE===========---")
print('--------------', (Sd), 'meters', '------------')

Answer 1

• Some of the numbers might seem plucked out of thin air but they aren't

  Still better have them explained. I can understand 3.6 and even 19.6 (it is \$2g\$, isn't it?), but I have no idea what 98/490 stands for. Declare them as symbolic constants with meaningful names.

• I don't see a need for abs in

      reaction_time = round(abs(diff.total_seconds()), 2)
  

  Here diff may not be negative. If it is, I'd rather throw an exception, because the result is meaningless anyways.

• You are rounding too early. There is no penalty computing Rt with reaction time not being rounded, but rounding leads to loss of precision. Only round when printing results.

• You have too many parenthesis. Sd = Bd + Rd is much cleaner than Sd = ((Rd)+(Bd)).

Answer 2

I suggest breaking the code into functions, so you can easily import your code into a Python shell and test that your computation is exact (and not just thinking it is).

To expand on other answers, you may:

• retry asking the speed if it is not an integer (detected when the int function raises a ValueError);
• in the same vein, since you don't really need to check for both wet and dry, you could ask a simple yes/no question;
• flush the input stream before printing GO (using termios on linux or msvcrt on windows);
• prefer time.perf_counter() to datetime.now() to compute the time elapsed between two points in the program;
• compute all the results before printing them.

Proposed improvements:

import random
import time


def flush_input():
    try:
        import msvcrt
        while msvcrt.kbhit():
            msvcrt.getch()
    except ImportError:
        import sys, termios    #for linux/unix
        termios.tcflush(sys.stdin, termios.TCIOFLUSH)


def ask_speed():
    while True:
        speed = input("Speed in km/h: ")
        try:
            return int(speed)
        except ValueError:
            print("An integer is required!")


def ask_is_wet():
    is_wet = input("Is the road wet? [y/n] ")
    return is_wet.lower() in ('y', 'yes')


def compute_distances(speed_kmh, reaction_time, is_wet):
    speed_ms = speed_kmh / 3.6
    friction_coefficient = 0.4 if is_wet else 0.6

    reaction_time = (reaction_time * 98 / 490)**.5
    reaction_distance = reaction_time * speed_ms
    braking_distance = speed_ms**2 / (19.6 * friction_coefficient)
    total_distance = reaction_distance + braking_distance

    return reaction_time, reaction_distance, braking_distance, total_distance


def get_reaction_time(prompt="GO!! "):
    time.sleep(random.randint(1, 12))
    then = time.perf_counter()
    flush_input()
    input(prompt)
    now = time.perf_counter()
    return now - then


def main():
    name = input("Player one: ")
    speed = ask_speed()
    is_wet = ask_is_wet()
    time.sleep(0.5)
    print("Get ready..")
    reaction_time = get_reaction_time()

    reaction, distance, braking, total = compute_distances(speed, reaction_time, is_wet)

    print('Your final reaction time was: ', reaction, '!\n')
    print('Your final reaction distance was: ', distance, '!\n')
    print('Your final braking distance was: ', braking, '!\n')
    print("\n\n---===========FINAL STOPPING DISTANCE===========---")
    print('--------------', total, 'meters', '------------')


if __name__ == '__main__':
    main()

Answer 3

• It crashes if user input for speed_ms is not integer
• If user presses any key before t = input("GO!! ") his reaction time is saved as 0. This is a major flaw in a code, and should be fixed.

Answer 4

If it's wet, the friction_coefficient will be lower. Which means that Bd will be lower (which is probably wrong). It's difficult to see that, because of all the unnecessary parentheses. You missed the necessary ones. You probably want

Bd = speed_ms**2 / (19.6 * friction_coefficient)

Rt, Rd, and Bd should get better names, or need a comment. In particular Rt, which probably means reaction time -- there already is a name reaction_time with a perfectly sensible value. That Rt contains a sqrt is suspicious. It needs a comment, if it is correct.

Answer 5

• player_1 is read and assigned but never used.

• If the user misspells wet (e.g. as Wet or as wet), the program assumes that dry was meant - it should ask again until it gets an answer it understands or the player gives up (and consider adding icy for more fun).

• There's a lot of unnecessary parentheses - they could be trimmed to make the code more readable.

Answer 6

Don't rely on users following your instruction to enter in lower case. Do conditions = input("Wet or dry conditions? (all lowercase)").lower(). Instead of doing if else, check each possibility separately, and ask again if it's neither (you might want to do 'wet' in conditions rather than `'wet' == conditions in case they put an extra space at the end or something).

Since you're going to divide by a non-integer, you're going to end up with float anyway, so it's probably better to cast speed as a float to begin with, and avoid rounding if they don't enter an integer. You should also do a try block to catch people entering non-numbers.

Answer 7

Output formatting: Your print calls output spaces between the comma-separated arguments; this causes output like:

Your final reaction time was:  1.6 !

Those extra spaces are ugly. An easy fix is the sep argument for print:

print('Your final reaction time was: ', (Rt), '!\n', sep='')

Your final reaction time was: 1.6!

This will print no spaces between the three parts. Alternatively, you could do some string concatenating, but I prefer the first approach.

I would also drop the colon and include a unit for this measurement:

Your final reaction time was 1.6 seconds!

_{Slowest gun in the west...}